You're absolutely correct. What they don't tell you is that FAR 121.471(c) translates to my point of 10 hours compensatory rest after a 15 hour day. Do you remember the 24 hour look back? Anytime you complete a flight, you have to ask yourself if you've had normal rest (9hours) in the last 24 hours. Normally the answer is yes. But, when you go over 15 hour duty day, the answer is no. You've had less than nine. Meaning, you had reduced rest in the last 24 hours, which means you have to have compensatory rest (10 hours). Remember the Whitlow letter? I don't have it in front of me, but I do know that it is about all this.
This is a very confusing subject, much more when there's a moron like me trying to explain it. Call Scheduling Hotline, he'll be glad to help you. The thing is that it does not say anywhere - "15 hour day = 10 hour rest". Unfortunately, we have to do the math. If I'm wrong on this, feel free to correct me.